This invention relates to directional couplers and more particularly to four port directional couplers using multiple transformers.
A directional coupler is an electrical device that accepts high frequency signals from a high frequency source and transmits most of the high frequency power associated with the high frequency signal to its output terminals while providing a considerably smaller portion of the high power energy on its tapped outputs.
U.S. Pat. No. 3,776,499 disclosed a coupler arrangement that has an input, a main line output and a tapped output. A ferromagnetic core has a first and second axial bore. A first winding passes through the first bore and has one end connected to the input and a second end connected to the main line output. A second winding has a first terminal connected to the input, passes through the second bore and has a second terminal connected to ground. A third winding has a first end portion connected to the tapped output, passes through the second bore and has a second end portion connected to ground through a terminating resistor. A fourth winding has a first end portion connected to the tapped output, passes through the first bore and has a second end portion connected to ground potential. The first and fourth windings are wound through the bore and around the outside portion of the core in a first direction, while the third and fourth windings are wound in the opposite direction. The first and third windings have a number of turns which are much smaller than the number of turns of the second and fourth windings.
A broadband directional coupler is disclosed in U.S. Pat. No. 4,121,180 in which a four port directional coupler using two substantially identical transformers, each transformer having a turn ratio N with a substantially pure resistive impedance across both the transformer's secondary windings, and a substantially pure resistive impedance connecting the transformer's primary windings, whereby the coupling coefficient, C, of the directional coupler is a function of the resistive impedance and N is substantially a continuous function of the values of said resistive impedance for any independent selected value of N.
The prior art directional couplers, however, do have some limitations in the area of directivity. Directivity is a quality of merit of a directional coupler and indicates the ability of a directional coupler to separate forward power from reflected power.